show start_at, end_at for conferences api
show start_at, end_at for conferences api

flag=none

TestPlan:
- go to: /api/v1/courses/:course_id/conferences
- you can see before and after patchset, so
 long as you have at least one conference.
- verify you see start_at and end_at

Notes:
- This is the route the api uses, you can use via browser.
- These fields are used with calendar_event
Refs:
 https://gerrit.instructure.com/c/canvas-lms/+/27592

Change-Id: Ibb69d83514b0d5820b8b68378256c552bd1f5b32
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/300888
QA-Review: Jason Gillett <12d37bea14b8257b31fd2f970455158adbd46adb@instructure.com>
Reviewed-by: Drake Harper <798ef92bc86a79e8ff768a68dc0a5fcec1d51660@instructure.com>
Product-Review: Drake Harper <798ef92bc86a79e8ff768a68dc0a5fcec1d51660@instructure.com>
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>

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